1. Technical Field
The present invention relates to a charge-controlling system and a method therefor. More particularly, the present invention relates to a charge-controlling system and a charge-controlling method applicable to a backup power system.
2. Description of Related Art
A fuel cell is a cell that converts chemical energy into an electric output by means of an electrochemical reaction, and it works on the principle that a fuel containing hydrogen and an oxidant (air or oxygen) are delivered to an anode and a cathode of the fuel cell, wherein the fuel is decomposed into hydrogen ions and electrons at the anode, and the hydrogen ions are transferred to the cathode from the anode through a proton exchange membrane to be react with electrons delivered to the cathode by an external circuit so as to generate water. By consistently providing the fuel, the fuel cell is enabled to continuously generate electric power. Since the reaction product of the fuel cell is water, it brings no pollution to the environment. As being highly effective and free from pollution, this technology has long been extensively followed with interest since its launch.
Power supplying of such a fuel cell is dependent on fuel concentration, reaction temperature, fuel delivery and movement of electron currents, so a fuel cell's output voltage and current are highly subject to its load device. When the dynamic load device requires power of high current at a transient time, it is necessary to reduce the reaction voltage of the fuel cell immediately, so as to provide the current required by the load. However, due to the reaction mechanism of the fuel cell, it is not easy to provide the load with transient high-power electric energy. In addition, in the course of repeated power generation, the fuel cell tends to have the generated power transiently unstable because of the concentration decay of the fuel.
Traditionally, in order to prevent transient high-power demand caused by load variation or to prevent unstable power generated by the reaction of the fuel cell, a capacitor or a secondary cell set may be used with the fuel cell. Therein, the secondary cell set maybe charged by constant-voltage charging or constant-current charging.
When charged by constant-voltage charging, the secondary cell set, at the initial stage of charging, has its voltage relatively low, so the excessively large charging current can heat and in turn damage the secondary cell set.
When charged by constant-current charging, the secondary cell set is free from the excessively large charging current at the initial stage of charging because the charging current does not vary with the increasing voltage of the secondary cell set. However, at the late stage of charging, the charging voltage is likely to go beyond the limitation of the secondary cell set and thus damage the secondary cell set. Furthermore, since the charging current is always constant, a relatively long charging time would be required.